Hasn’t Apple already shown it’s possible to add another party silently to an encrypted exchange?

> Passwords: Users can now create a group to share a set of passwords. Everyone in a group can add and edit passwords to keep them up to date, and since sharing is through iCloud Keychain, it’s end-to-end encrypted.

So there goes the backdoor encryption argument, it’s possible to add another member to a group of people with on device access?